ORGANIC REACTIONS
D. F. Palacio, LPT, MA.
Learning Outcomes
• At the end of the lesson, students are able to:
- Identify and discuss the common reactions in
organic chemistry
- Predict the major and minor product of an organic
chemical reaction
- Classify organic compounds based on reactivity
- Identify the common organic reactions that
happen in biological systems
Organic Reactions
• Reactions are the heart of organic chemistry
• It usually begins with electron-rich or deficient sites
at functional groups in the reacting molecules.
• These are often the location of bonds that might be
easily broken.
Writing Equations for Organic Reactions
• Equations for organic reactions are usually drawn with
a single reaction arrow (→) between the starting
material and product.
• The reagent, the chemical substance with which an
organic compound reacts, is sometimes drawn on the
left side of the equation with the other reactants.
• At other times, the reagent is drawn above the arrow.
• Although the solvent is often omitted from the
equation, most organic reactions take place in liquid
solvent.
Ways to Write Organic Reactions
• The solvent and temperature of the reaction may be
added above or below the arrow.
• The symbols “h” and “” are used for reactions that
require light or heat, respectively.
• When two sequential reactions are carried out, the
steps are numbered above or below the reaction
arrow.
• This convention signifies that the first step occurs
before the second step, and the reagents are added
in sequence, not at the same time.
Substitution Reaction
• Substitution is a reaction in which an atom or a
group of atoms is replaced by another atom or
group of atoms.
• In a general substitution, Y replaces Z on a carbon
atom
• Substitution reactions involve  bonds: one  bond
breaks and another forms at the same carbon
atom.
• While in some cases Z can be a hydrogen atom, the
most common examples of substitution occur
when Z is a heteroatom that is more
electronegative than carbon.
Elimination Reactions
• Elimination is a reaction in which elements of the
starting material are “lost” and a  bond is formed.
• In an elimination reaction, two groups X and Y are
removed from a starting material.
• Two  bonds are broken, and a  bond is formed
between adjacent atoms.
• The most common examples of elimination occur
when X = H and Y is a heteroatom more
electronegative than carbon.
Addition Reactions
• Addition is a reaction in which elements are added
to the starting material.
• In an addition reaction, new groups X and Y are
added to the starting material.
• A  bond is broken and two  bonds are formed.
Alkanes
• Combustion: Reaction of alkanes with oxygen to
produce carbon dioxide, water, and heat
• Halogenation: Reaction of alkane with halogens to
produce alkyl halides
Alkenes
• Reactions of alkenes involves the C=C bond,
• Addition of 2 atoms or groups of atoms to a double
bond reaction is the key bonding
• Hydrogenation – the addition of hydrogen atoms
• Halogenation – the addition of halogen atoms
• Hydration – the addition of a water molecule
• Hydrohalogenation – the addition of a hydrogen halide
molecule
Addition: General Reaction
• A small molecule, AB, reacts with the pi electrons of the double bond
• The pi bond breaks and its electrons are used to bond to the A and B
pieces
• Some additions require a catalyst
Types of Addition Reactions
1.
Symmetrical: same atom added to each carbon
• Hydrogenation - H2 (Pt, Pd, or Ni as catalyst)
• Halogenation - Br2, Cl2
2.
Unsymmetrical: H and another atom are added to the two
carbons
• Hydrohalogenation - HCl, HBr
• Hydration - H2O (requires strong acid catalyst e.g., H3O+, H2SO4,
H3PO4)
3.
Self-addition or polymerization
Hydrogenation: Addition of H2
Hydrogenation is the addition of a molecule of hydrogen (H2)
to a carbon-carbon double bond to produce an alkane
•
•
•
•
The double bond is broken
Two new C-H bonds result
Platinum, palladium, or nickel is required as a catalyst
Heat and/or pressure may also be required
Halogenation: Addition of X2
Halogenation is the addition of a molecule of halogen (X2) to a carbon-carbon
double bond to produce an alkane
•
The double bond is broken
•
Two new C-X bonds result
•
Reaction occurs quite readily without a catalyst
•
Cl and Br are most often the halogen added
Hydration
A water molecule can be added to an alkene
• The addition of a water molecule to an alkene is called hydration
Presence of strong acid is required as a catalyst
Product resulting is an alcohol
Unsymmetrical Addition
Two products are possible depending how the 2 groups (as H and OH) add to
the ends of the pi bond
•
The hydrogen will add to one carbon atom
•
The other carbon atom will attach the other piece of the addition reagent
•
OH (Hydration)
•
Halogen (Hydrohalogenation)
Markovnikov’s Rule
• Dimitri Markovnikov (Russian) observed many acid additions
to C=C systems
• When an acid adds to a double bond, the H of the acid most
often goes to the end of the double bond, which had more
hydrogens attached initially
• H-OH
• H-Cl
• H-Br
Hydrohalogenation
An alkene can be combined with a hydrogen halide such as HBr or HCl
The reaction product is an alkyl halide
Markovnikov’s Rule is followed in this reaction
Polymerization
• Reaction of alkene with hydrogen to create
polymers
• Polymers: A type of long-chain macromolecule linked
together by repeated monomers (one unit of organic
molecule
Some Important Addition Polymers of
Alkenes
Monomer name
Polymer
Uses
Styrene
Polystyrene
Styrofoam
Acrylonitrile
Polyacrylonitrile (Orlon)
Clothing
Methyl
methacrylate
Polymethyl methacrylate
(Plexiglas, Lucite)
Basketball backboards
Vinyl chloride
Polyvinyl chloride (PVC)
Plastic pipe, credit cards
Tetrafluoroethene
Polytetrafluoroethylene
(Teflon)
Nonstick surfaces
Uses of Polymers
Uses of Polymers
Aromatic Compounds
•
The typical reaction is the substitution reaction- hydrogen is
replaced by another atom or group of atoms
• All benzene reactions we consider require a catalyst
• The reactions are:
1. Halogenation
2. Nitration
3. Sulfonation
Halogenation
A reaction that places a Br or Cl on the ring
• The reagent used is typically Br2 or Cl2
• Iron or iron halide are used as catalysts
Nitration
• A reaction that places the nitro group on the
ring
• Sulfuric acid is needed as a catalyst
Sulfonation
Sulfonation places an SO3H group on the ring
• Concentrated sulfuric acid is required as a catalyst
Alcohols
• Acid-catalyzed Dehydration: Elimination of water molecule
from alcohols through the aid of concentrated acid and heat
as a catalyst to yield alkenes
Alcohols
• Oxidation: Primary alcohols usually oxidize to
carboxylic acids
• The symbol [O] represents an oxidizing agent
(KMnO4/OH- or H2CrO4) which is used to oxidize a
1 degree alcohol to an aldehyde
• Oxidation of primary alcohols
• Oxidation of secondary alcohols
• Secondary alcohols oxidize to ketones
• This reaction is also an elimination of 2H
• Tertiary alcohols do not oxidize as there is no H on
the carbonyl carbon to remove
Oxidation and Reduction in Living
Systems
• Oxidation is the loss of electrons
• Reduction is the gain of electrons
• These changes are easily detected in inorganic systems with
formation of charged ions
• In organic systems it is often difficult to determine whether
oxidation or reduction has taken place as there might be no
change in charge
Organic Oxidation and Reduction
In organic systems changes may be tracked:
Oxidation:
• gain of oxygen
• loss of hydrogen
Reduction:
• loss of oxygen
• gain of hydrogen
Biological Oxidation-Reduction
NAD+ is a coenzyme commonly involved in biological
oxidation/reduction reactions:
Oxidation and Reduction in Living
Systems
Thiols
• Disulfide
Formation
• Two cysteine
molecules
(amino acids)
can undergo
oxidation to
form cystine
• Forms a new
bond called a
disulfide bond
• Play a crucial
part in proteins
Activity
Predict the major product in each of the following reactions
Name the alkene reactant and the product using IUPAC
nomenclature